Noise and shock reduction in rotary positive displacement blowers

ABSTRACT

A roots-style blower ( 210 ) has a housing ( 214 ), rotors ( 230 ) and manifolds ( 212 ) for the housing ( 214 ). The housing ( 214 ) forms an inlet plenum ( 220 ), a rotor chamber ( 224 ) and a discharge plenum ( 228 ). The rotors ( 230 ) have straight lobes ( 232 ) spaced by pockets ( 240 ). The pocket ( 240 X) that traps gas between a leading and following lobe ( 232 ) and an inside wall of the rotor chamber ( 224 ) is a temporary closed cell ( 240 X). The manifolds ( 212 ) and the housing ( 214 ) form a pair of back-pass loops ( 250 - 251 ), one for each rotor ( 230 ). Each back-pass loop ( 250 - 252 ) comprises a back-pass chamber ( 250 ), outer channels ( 252 ) from the discharge plenum ( 228 ) to the back-pass chamber ( 250 ), and inner channels ( 251 ) to the rotor chamber ( 224 ). Wherein, the back-pass chamber ( 250 ) volume as a percentage of closed cell ( 240 X) volume ranges between about fifty-six percent (56%) and one-hundred-seventeen percent (117%).

CROSS-REFERENCE TO PROVISIONAL APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.61/336,495, filed Jan. 22, 2010, the disclosure of which is incorporatedherein by this reference thereto.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to rotary positive displacement blowers (of theRoots type) and, more particularly, to a back-pass loop for graduallypressurizing the working cell to outlet pressure in order to weaken thestrength of the pulsations that would otherwise happen without such aback-pass loop, and thereby reduce noise and shock (and perhaps betterefficiency as well).

Briefly, the performance of such blowers is typically measured (orspecified) in terms of the following factors:—flow, pressure,efficiency, noise, and reliability.

It is an object of the invention to provide improvements in particularfor at least two or three of those factors, namely, noise andreliability, plus perhaps efficiency.

Although the invention perhaps neither betters nor harms flow rate andpressure performance to a significant degree, to be sure, these areimportant factors to users.

So, briefly (and very briefly), the following remarks are offered aboutflow and pressure. Regarding flow rate, blowers of this type can bebuilt to all kinds of sizes (including very large). Hence design flowrate is an operating point that is scalable over a wide range.

As for pressure, the operating pressure differential (Δp) across suchblowers might typically vary under the circumstances between very slight(eg., 1 to 2 psi or ^(˜) 1/15th to 2/15th atm) to something typical(eg., 15 psi or ^(˜)1 atm). It might be just as typical that a blower ofthis type be rated for up to 18 psi duty (^(˜)1 3/15ths atm pressuredifferential).

As concerns a separate consideration, some end-use applications mayrequire that the discharge line supply flow at a pressure as high as 100psig (^(˜)7⅔rds atm). To do this, the pressure in the inlet line has tobe elevated to within 18 psi (^(˜)1 3/15ths atm pressure differential)or less of the target pressure for the discharge line.

Moreover, high reliability is expected of these kinds of blowers. Theymight be designed and expected to operate more or less continuously(excluding routine maintenance) for years on end.

This application is owned by assignment in common with the same owner ofU.S. Pat. No. 5,702,240—O'Neal et al., namely TUTHILL CORPORATION ofBurr Ridge, Ill. This blower was referred to by the TUTHILL CORPORATIONas the “Acoustic Air” design.

The Acoustic Air blower introduced some matters in blower design whichhave been changed, substantially or so, here for better meeting theobjects of the invention. These changes fall under two major categories.One major category comprises changes in design for purely orsubstantially pneumatic reasons. The other major category compriseschanges in design for purely or substantially ease of manufacturereasons.

In common with one of the objects of the invention here, an object ofthe invention for the Acoustic Air design included reducing pressurepulsations, and thereby reducing resulting noise and vibration.

The Acoustic Air design sought to do this by the following two ways.One, the Acoustic Air design included a backflow loop. Generallyspeaking, a backflow loop is meant to gradually pre-pressurize alow-pressure closed cell (eg., 64 or 66) so that when the closed cell(eg., 64 or 66) opens across an edge 78 or 80 into the higher-pressuredischarge chamber 46, the backflow loop eliminates or weakens the directbackflow from the discharge chamber 46 into the opening closed cell(eg., 64 or 66). Without a backflow loop, the backflow from thedischarge chamber 46 flows directly into the opening closed cell (eg.,64 or 66) and is the source of the sonic pop (eg., the noise) as well asthe momentary opposition to the rotation of the rotors 50, 52 (eg., thevibration).

With reference to FIGS. 3 and 6 therein, the Acoustic Air blower hasbackflow chambers 106, 108, 120, 122 filled by backflow ports 112, 116,126, 130 and for pre-pressurizing fluid in the sealed pocket (eg., 64,66) by injector ports 110, 114, 124, 128. The patent contains thisremark on the effectiveness of this design.

-   -   . . . Therefore, after a pocket 64 has been in fluid        communication with the injector ports 110 and 114, the pressure        in the now pre-pressurized pocket 64 is greater than the first        pressure of the fluid within the intake chamber 44, but is        usually still somewhat lower than the second pressure of the        fluid within the discharge chamber 46. U.S. Pat. No. 5,702,240,        col. 6, lines 28-34.        In other words, the backflow loop was not as effective as hoped        for. The other way the Acoustic Air design sought to eliminate        or weaken backflow was by curved edges 78 and 80 opening into        the discharge chamber 46.

Despite owning the rights to the Acoustic Air design, the owner of thepatent thereon put together the present team of inventors to do evenbetter. Noise and vibration are serious problems. It is an object of theinvention to overcome the shortcomings of the prior art.

Now to turn to the improvements herein, various features and objects ofthe invention will be apparent in connection with the followingdiscussion of preferred embodiments and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings certain exemplary embodiments of theinvention as presently preferred. It should be understood that theinvention is not limited to the embodiments disclosed as examples, andis capable of variation within the scope of the skills of a personhaving ordinary skill in the art to which the invention pertains. In thedrawings,

FIG. 1 is a perspective view of a rotary positive displacement blowerwith noise and shock reduction improvements in accordance with theinvention;

FIG. 2 is an exploded view thereof;

FIG. 3 is an enlarged scale detail view of the rotor chamber anddischarge plenum in FIG. 2;

FIG. 4 is a vertical sectional view taken through the rotary positivedisplacement blower of FIG. 1, taken perpendicular to the plane of therotor axes, and, taken along an offset plane to contain not only thecenterline of one inner port(s) in the rotor chamber (as well asportions thereabove), but also, the centerline of one (of the two) outerport(s) in the discharge plenum (as well as portions therebelow); and

FIG. 5 is a chart showing the effect of back-pass manifold chambervolume on the fluctuation away from mean discharge flowrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 4 provide line drawings of a rotary positivedisplacement blower 210 with noise and shock reduction improvements inaccordance with the invention. It is an aspect of the invention toincorporate a pair of back-pass manifolds 212.

This is a Roots style blower. FIG. 4 shows better that, it has asubstantially hollow housing 214 defining an inlet plenum 220, a rotorchamber 224, and a discharge plenum 228. (Preferably the housing 214 iscast, but the flange surfaces would be machined and ground.)

A pair of rotors 230 are disposed in the rotor chamber 224. The rotors230 would be sealed inside by a pair of opposed end plates (far side endplates shown in FIGS. 1 and 2). The rotors 230 are driven to rotatecounter-rotationally to each other. For instance, the left rotor 230rotates counter-clockwise (CCW).

In the drawings, the blower 210 is shown with the inlet port 220P up andthe discharge port 228P down. However, the blower 210 can be mounted inany orientation, and accordingly, terms like “up” and “down”, “left” and“right” are used merely for convenience in this description and do notlimit the installation of the blower 210 to any particular orientation.

The rotors 230 are identical. Each rotor 230 comprises three lobes 232.Each lobe 232 culminates in a tip 232T. The lobes 232 are spaced bypockets 240.

The inlet plenum 220 transitions into the rotor chamber 224 at a pair ofspaced ledges 242L, and these define an inlet opening 242 for the blower210. Likewise, the rotor chamber 224 transitions into the dischargeplenum 228 at another pair of spaced ledges 244L, and these define adischarge opening 244 for the blower 210. FIG. 4 shows that the leftrotor 230's upper lobe tip 232T is about to sweep (counterclockwise)past the left ledge 242L of the inlet opening 242. When it does so, thatlobe 232 will trap gas in the pocket 240X immediately ahead of it,between the surface of the rotor 230 and surface of the housing 214. Thepocket 240X which temporarily traps gas in it, carrying the trapped gasfrom the inlet plenum 220 to the discharge plenum 228, is referred to asthe ‘closed cell’ (ie., indicated as 240X). Each pocket 240 in turn willform the temporarily existing closed cell 240X, successively, and in anendless succession.

The trapped gas is carried around in the closed cell 240X, from theinlet plenum 220 to the discharge plenum 228, at the pressure of theinlet plenum 220 while being carried around like that. In contrast, thetrapped gas will be ultimately discharged into the discharge plenum 228,at the pressure of the discharge plenum 228.

When the lobe tip 232T of the lobe 232 leading the closed cell 240Xsweeps past the ledge 244L of the discharge opening 244, suddenlysomething happens. Two different pressurized spaces at two differentpressures have open communication with each other. This allows for thefree exchange of gases between the (formerly) closed cell 240X and thedischarge plenum 228. This ‘opening’ of the (formerly) closed cell 240Xto the discharge plenum 228 also allows for the consequentialequalization of pressure between the two. That is, the closed cell 240X,as it travels from inlet space to discharge space, holds fairly steadyat the inlet pressure. But that changes, suddenly, when the lobe tip232T of the leading lobe 232 crosses the ledge 244L of the dischargeopening 244. At that moment, the closed cell 240X is suddenly no longerclosed but ‘open’ to the discharge plenum 228. Gases in the dischargeplenum 228 are free to flow back into the (formerly) closed cell 240X.

There are numerous consequences to this ‘moment’ that the closed cell240X opens to discharge space. There is noise (eg., an audible sonic popor snap, something akin to a popping balloon or snapped cell of bubblewrap), and there is a puff of reverse flow from the discharge plenum 228into the opening closed cell 240X. Noise aside (for the moment), thereverse flow is a problem of its own. The reverse flow creates anopposing force in opposition to the turning rotors 230, and the rotors230 have to power through the reverse flow. Hence the reverse flow is areadily identifiable source of inefficiency. The reverse flow also hasanother effect, which is likewise detrimental, which is that of causingmechanical shock through the blower (vibration), and not just to theblower's castings but also to the joints, couplings, bearings, seals andso on.

To come to terms with the problematic effects of reverse flow, it paysto appreciate that the reverse flow comprises a pulsing phenomenon. Thatis, for each revolution of the rotors 230, there are six reverse flowevents. The rotors 230 are typically driven at 1200, 1800 or 3600 RPM.At the high value given there, that corresponds to 1.3 million reverseflow pulses—each hour.

Hence the effects of reverse flow comprise an unceasing hammering on theblower, and over its whole lifetime. Accordingly, it is an object of theinvention to not just weaken but eliminate each reverse flow event. Itis a further object of the invention to reduce vibration, and not somuch the frequency of the vibration but the shock value (amplitude) ofeach pulse. It is a corresponding object of the invention to enhancereliability.

These and other objects and aspects are provided according to theinvention in a rotary positive displacement blower 210 (of the Rootstype) with a back-pass loop 250-52 for gradually pressurizing the closedcell 240X to the pressure of the discharge plenum 228 in order to weakenthe strength of the pulsations that would otherwise happen, and therebyreduce noise and shock.

FIGS. 1 and 4 show a rotary positive displacement blower 210 providedwith a pair of flanking manifolds 212. In FIG. 2, both manifolds 212 areshown dismounted and apart from the main housing 214. Conversely inFIGS. 1 and 4, both manifolds 212 are shown mounted to the main housing214.

Just as the main housing 214 is a monolithic casting of (preferably)steel, so is each manifold 212 its own separate monolithic casting ofsteel. The flange surfaces for the bolt-on surfaces are preferablyground very smooth, as are the mating surfaces on the main housing 214.

FIGS. 1 and 2 allow discernment that the manifolds 212 mount to the mainhousing 214 by a pattern of bolts (bolts not shown). Each manifold 212defines a back-pass chamber 250.

The main housing 214 is bored through from both sides in order to form anumber of channels 251 and 252 for connecting each back-pass chamber 250into a back-pass loop 250-52 with the blower 210. That is, the mainhousing 214 is bored through a series of times into each side of therotor chamber 224 to form a pattern—a line parallel with the axis of therotor 230—of inner channels 251 to the rotor chamber 224. The mainhousing 214 is furthermore bored through two times into each side of thedischarge plenum 228 to form a pattern of (eg., two in-line) outerchannels 252 (‘outer’ relative to the rotor chamber 224). FIG. 3 showsbetter the ports 251P and 252P of the inner and outer channels 251 and252, respectively, in the rotor chamber 224 and discharge plenum 228,respectively.

It is a design preference at present time that the cumulativecross-sectional flow area for the two outer channels 252 feeding onemanifold 212 chamber 250 equals or is substantially close in value tothe cumulative cross-sectional flow area of all the inner channels 251serving the same manifold 212 chamber 250. Hence for each manifold 212chamber 250, the ratio of the cumulative cross-sectional area of theouter channels 251 to that of the inner channels 251 is about one to one(1:1).

FIG. 4 shows better that the flow axis of gas through the blower 210 isgenerally perpendicular to the plane containing the rotor axes. Thisplane (that contains the rotor axes) is referred to herein forconvenience sake as the rotor plane. (It might alternatively be referredto as the dowel plane. As FIG. 1 shows better, it is typical that ahousing 214 for a Roots blower would contain a pair of flanking dowels255 in this same plane. These dowels 255 provide for alignment to theend plates and support to the housing 214 in this plane, and hencepromote proper lobe tip 232T clearance.)

Given the foregoing, the manifolds 212 mount to the main housing 214 onthe discharge side of the rotor plane.

FIG. 4 allows reckoning of the following matters. The lobes 232 of therotors 230 are angularly spaced apart by 120°. The ledge 242L of theinlet opening 242 and the ledge 244L of the discharge opening 244 areangularly spaced apart by about 180° (relative to rotor rotation).

Hence, in the absence, of the improvements of the invention, thetemporarily existing closed cell 240X is formed for a time periodcorresponding to a 60° arc of the rotor rotation. In other words, thereis a window of opportunity during that 60° arc in which to graduallypressurize the closed cell 240X from inlet pressure to dischargepressure.

It is an object of the invention to gradually pressurize the closed cell240X from inlet pressure to discharge pressure over the last 30° to 40°or so of rotation of the closed cell 240X to its opening to thedischarge opening 244.

The design in accordance with the invention was obtained by virtualprototyping with the use of three-dimensional CFD software fromSIMERICS, INC., that goes by the brand name PUMPLINX®.

The CFD analysis was performed with an existing blower of TUTHILL VACUUM& BLOWER SYSTEMS, model QX-3208, serving as the basis for blowerdimensions. The operating point for the analysis was chosen to be 3600RPM at 15 psi (^(˜)1 atm pressure differential).

Following that, a physical prototype was built, and tested, at thefollowing operating points:

-   -   1200 RPM @ 10 psig (^(˜)⅔rds atm pressure differential).    -   1800 RPM @ 10 & 15 psig (^(˜)⅔rds and 1 atm pressure        differential).    -   3600 RPM @ 10, 15 & 18 psig (^(˜)⅔rds, 1 and 1 3/15ths atm        pressure diff.).

At the CFD operating point of 3600 RPM at 15 psig (^(˜)1 atm pressuredifferential), the prototype blower 210 in accordance with the inventioncompares to the un-modified original QX-3208 as follows. There was 8.9db drop and a 12.4 dBA drop in sound pressure levels. There was anaverage drop across all tested speeds and pressures of 7.4 dB and 10.7dBA. The maximum sound pressure level drop was 3600 RPM and 18 psi(^(˜)1 3/15ths atm differential pressure) for both linear and A-weightedscales. These results were 13.2 dB and 17.1 dBA respectively.

(Note: Sound pressure levels were recorded by four microphones locatedon the horizontal plane bisecting the blowers—the rotor plane—locatedsix inches or roughly 15 cm from the corners of the main housings andcentered on axis passing through the inlet and discharge ports.)

There is a noticeable difference in not just the quieting of the soundof the blower 210 in accordance with the invention, but also the qualityof the sound. Indeed, there are still personnel employed by TUTHILLVACUUM & BLOWER SYSTEMS who can personally recall the Acoustic Airblower referenced above in connection with U.S. Pat. No. 5,702,240. Onesuch person includes one of the original inventors. The remarks aboutthe change in sound quality with the blower 210 in accordance with theinvention is something as follows:—the blower 210 in accordance with theinvention is not just merely a quieter jack hammer, it has sort of lostits jack hammer staccato to where it just sounds like the hum of processmachinery.

The CFD analysis in combination with building and testing a number ofprototypes discovered that perhaps the following five (5) factors arechiefly responsible for the blower 210 in accordance with the inventionworking so well.

These five (5) factors include the following:—

-   -   1—ease of manufacture,    -   2—cumulative flow area of inner channels 251,    -   3—angle of attack of (and like matters with) the inner channels        251,    -   4—timing, or separation between plane of the outer channels 252        and plane of the discharge opening ledges 244L, and    -   5—ratio of manifold chamber 250 volume to closed cell 240X        volume.

(1) To begin with, a nod is given to ease of manufacture as an importantfactor. The improved blower 210 was prototyped out of a stock QX-3208blower of TUTHILL BLOWER & VACUUM SYSTEMS. The casting of the stockblower had to be beefed up in the regions where the inner and outerchannels 251 and 252 were to be drilled, as well as where the manifolds212 bolt on. However, the manifold 212 is its own casting. In theAcoustic Air blower, the backflow chambers were cast to size in the mainhousing casting for the blower. In accordance with the invention, themethod of manufacture of the blower 210 with its separate cast manifolds212 allowed much more flexibility in specifying different sizes andarrangements of inner and outer channels 251 and 252 as well a volume ofthe manifold 212 chambers 250.

(2) The second important factor is the cumulative flow area of the innerchannels 251. With a given back-pressure in the manifold 212 chamber 250and under-pressure in the closed cell 240X, the cumulative flow area isselected to fill the closed cell 240X with about 100% plus of themake-up mass of air in the angular time that the inner channels 251 arefilling the closed cell 240X (eg., about 30° to 40° angular degrees). Itis preferred that the outer channels 252 cumulatively form about thesame cross-sectional flow area for each manifold 212 chamber 250 as dothe inner channels 251 therefor. The inner channels 251 cannot beundersized or else there will be backflow when the leading lobe tip 232Tof the closed cell 240X crosses the discharge ledge 244L. Conversely,the inner channels 251 cannot be grossly oversized or else it just movesthe moment of backflow from • when the leading lobe tip 232T of theclosed cell 240X crosses the discharge ledge 244L to • when leading lobetip 232T of the closed cell 240X crosses the inner channels 251. In sum,the inner channels 251 have to fill gradually, and do so all the wayuntil the leading lobe tip 232T of the closed cell 240X crosses thedischarge ledge 244L, and then for a little while longer too.

(3) The third important factor is a series of factors, and comprises theangle of attack angle of, and like matters concerning the inner channels251. The angle of attack of the inner channels 251 is preferably is asclose to a tangent line with the curve of the rotor chamber 224 andblowing onto the backside of the leading lobe tip 232T of the closedcell 240X as it crosses the inner channels 251. Also, a prototype wasbuilt and tested where there were a series of inner channels 251 onthree lines. It is believed from that experiment that closed cell 240Xwants to open all the inner channels 251 on one line that is parallel tothe rotor axes. Hence the inner channels 251 preferably comprise aseries of same diameter bore holes equally spaced from one another andgenerously distributed along the axial length of the closed cell 240X inorder to fill the closed cell 240X in an axially even fashion.

(4) The fourth most important factor is a timing factor. Briefly, by wayof background, the pressure in the discharge plenum 228 oscillates. Theback-pass loop 250-252 goes a long way to dampening the fluctuations.But it does not flatten the fluctuations to zero. Indeed, modest to mildfluctuations are a good thing. The pressure fluctuations are propagatedat the plane of the discharge ledges 244L and move down (or away in) thedischarge plenum until eventually the pressure fluctuations have movedso far away from the plane of the discharge ledges 244L that they havecanceled each other out into a mean pressure (with no fluctuations). Butnear the plane of the discharge ledges 244L, there are measurablefluctuations. The timing issue relates to where to locate the outerchannels 252 relative to the plane of the discharge ledges 244L. FIG. 4illustrates where the outer channels 252 should be located. Given theright side of FIG. 4, it is preferred that a maximum of pressurefluctuation in discharge plenum 228 (even though propagated at the planeof the ledges 244L) should reside at the plane of the outer channels 252when the closed cell 240 on the right rotor 230 is about to cross theledge 244L. That way, the manifold 212 chamber 250 is pulling mass outof the discharge plenum 228 at the moment the closed cell 240 is aboutto blow out across the discharge ledge 244L, which will be experiencinga local minimum in the pressure fluctuation. By scaling the outerchannels 252 in connection with other proportions, the timing can bemanaged such that the opening closed cells 240/240X never experiencebackflow when crossing the ledges 244L.

(5) The fifth factor is left for last perhaps because its range was mostelusive. That is, it has been inventively discovered that theeffectiveness of the blower 210 in accordance with the invention issensitive to the ratio of closed cell 240X volume to manifold 212chamber 250. Moreover, it is believed to be highly preferable that therebe one dedicated manifold 212 chamber 250 pursuant to each rotor 230. Incontrast to the fourth factor above, the measure of performance here hasto do with flow fluctuations.

If the mean discharge flow rate is 100 feet per second (^(˜)30 m/s),then local flowrate at the plane of the discharge ledges 244Lfluctuates. How little it fluctuates is a measure of how effective theback-pass loop 250-252 is working. Recall that, in prior art blowerswithout a backflow loop or the like, the fluctuations can even gonegative.

FIG. 5 is a chart showing the effect of back-pass manifold 212 chamber250 volume relative to volume of the closed cell 240X on the fluctuationaway from mean discharge flowrate.

FIG. 5 shows that the best performance is obtained when manifold 212chamber 250 volume relative to closed cell 240X volume is 100% (eg., thevolumes are equal, or, there is one-to-one correspondence. Thefluctuation as a percentage of flowrate discharge is 12.8%. That meansthat, if the mean discharge centerline flowrate is 100 feet per second(^(˜)30 m/s), then the fluctuations in the flowrate are between about 93feet per second (^(˜)28 m/s) and 107 feet per second (^(˜)32 m/s).

FIG. 5 shows that when manifold 212 chamber 250 volume as a percentageof closed cell 240X volume is any of the following three values:—

-   -   83%,    -   56%, and/or    -   117%,        the fluctuation percentages of the discharge flowrate is still        believed to be within acceptable ranges of 14.6%, 14.8% and        17.5% respectively.

However, it is only when manifold 212 chamber 250 volume as a percentageof closed cell 240X volume is about 134% that the fluctuation percentageof the discharge flowrate is believed to have climbed to an un-preferredvalue of 25.3%

Given the foregoing, it is a preference of the invention that themanifold 212 chamber 250 volume as a percentage of closed cell 240Xvolume should fall between about 56% and 117% in order to obtain thepreferred performance of the blower 210.

One way to characterize how the back-pass loop 250-52 in accordanceimproves blower performance to the extent it does, might be thefollowing. The back-pass loop 250-52 weakens the pulsations by having anout-of-phase flow with chambers 250 comparable in volume to the closedcell 240X.

To turn to manufacture once more, it is an object of the invention toproduce the blower 210 in accordance with the invention fromconventional stock housings, except modified to accept the inventivemanifolds 212.

A preferred method of manufacturing a roots-style positive displacementblower 210 with a back-pass loop 251-252 comprises come of the followingsteps.

A housing 214 is provided, and it is highly preferred if the housing isa monolithic casting. The housing 214 has a rotor chamber 224 portion,an inlet plenum 220 portion defining an inlet plenum 220 and a dischargeplenum 228 portion.

The rotor chamber 224 portion defines a rotor chamber 224 comprisingside-by-side left and right cylindrical cavities partially overlappingone another and meeting at tangent lines. The left and right cylindricalcavities receiving the left and right rotors 230 such that the rotoraxes define a rotor plane.

The discharge plenum 228 portion comprises a bell shape extending alongan axis that projects away from the rotor plane. More preferred still isif the axis of the bell shape is perpendicular to the rotor plane. Thebell shape defines a discharge plenum 228 extending between a dischargeopening 244 in the rotor chamber 224 and a discharge port 228P.

The housing 214 is formed with left and right flange interfaces on therotor chamber 224 portion of the housing 214. Preferably this is done bysurface machining followed by grinding. The housing 214 is furthermoreformed with left and right inner channels 251 in the rotor chamber 224portion of the housing 214 that extend between interior ports 251P inthe left and right cylindrical cavities respectfully, and exterior portsin the left and right flange interfaces on the rotor chamber 224 portionof the housing 214.

Additionally, the housing 214 is preferably formed with left and rightflange interfaces of the discharge plenum 228 portion of the housing214, again as by surface machining and grinding. Then, the housing isformed with left and right outer channels 252 in the discharge plenum228 portion of the housing 214, which extend between interior ports 252Pin the left and right sides respectively of the discharge plenum 228,and exterior ports in the left and right flange interfaces of thedischarge plenum 228 portion of the housing 214.

It is an aspect of the invention to provide left and right ‘covers’ 212that removably attach to the housing 214 and cover portions of theflange interfaces on the rotor chamber 224 portion of the housing 214 aswell as portions of the flange interfaces of the discharge plenum 228portion of the housing 214 on the left and right sides respectively ofthe housing 214.

Wherein, these covers 212 concurrently seal over the exterior ports ofthe inner and outer channels 251 and 252, respectively, and allow aback-pass flow therebetween underneath said covers 212.

It is another aspect of the flange interfaces on the rotor chamber 224portion of the housing 214 that they are further outboard from the axisof the discharge plenum 228 than the flange interfaces of the dischargeplenum 228 portion of the housing 214. That way, the covers 212 might beL-shaped and still function sufficiently as covers 212.

As the drawings show, it is more preferential still that the ‘covers’212 are not just simply L-shaped by comprise a monolithic casting in atubular C-shape. Hence the ‘covers’ given the tubular C-shape mightinterchangeably be referred to as manifolds 212.

The manifolds 212 extend between a first interface for mating to theflange interfaces of the discharge plenum 228 portion of the housing 214and a second interface for mating to the flange interfaces of the rotorchamber 224 portion of the housing 214.

Each manifold 212 furthermore defines a back-pass chamber 250 whichallows the back-pass flow between the inner and outer channels 251 and252. It is an aspect of the invention that the manifolds 212 areremovably attached to the housing 214 by mechanical fastening.

It is another aspect of the invention that each manifold 212 expandsfrom being relatively narrower at the first interface to beingrelatively wider at the second interface. In this context, beingrelatively narrower and wider is taken in context along axes parallel tothe rotor axes.

Preferably the left inner channels 251 comprise a series of bore holesaxially spread apart on an axis parallel to the rotor axes (the rightinner channels 251 comprise symmetric opposites of the left innerchannels 251). Preferably the left outer channels 252 comprise at leasttwo bore holes axially spread apart on an axis parallel to the rotoraxes (right outer channels 252 would be symmetric opposites of the leftouter channels 252). That way, the spread of the manifold 212 couldaccommodate the spread apartness of the inner channels 251.

It is preferred again if the left inner channels 251 define a cumulativeflow area fairly close in size to the cumulative flow area defined bythe left outer channels 252.

As previously mentioned, it is preferred if the inventive housing 214 ismodified from conventional stock housings. Conventional stock housingare characterized by many design aspects, including without limitationthat the discharge plenum 228 portion of the housing 214 furthercomprises a circular ANSI flange encircling discharge port 228P, and thebell shape comprises a six-sided subtended diamond-shaped bell flare.

The invention having been disclosed in connection with the foregoingvariations and examples, additional variations will now be apparent topersons skilled in the art. The invention is not intended to be limitedto the variations specifically mentioned, and accordingly referenceshould be made to the appended claims rather than the foregoingdiscussion of preferred examples, to assess the scope of the inventionin which exclusive rights are claimed.

1. A roots-style positive displacement blower (210) comprising: ahousing (214) defining an inlet plenum (220), a rotor chamber (224) anda discharge plenum (228); a pair of rotors (230), each comprising atleast three axially-straight lobes (232), wherein each lobe (232)culminates in a tip (232T) and the lobes (232) are spaced by pockets(240); and wherein the pocket (240X) that traps gas between a leadingand following lobe (232) and an inside wall of the rotor chamber (224)is a temporary closed cell (240X); a pair of back-pass loops (250-251),one for each rotor (230); each back-pass loop (250-252) comprising amanifold (212) of the housing (214) formed with a back-pass chamber(250), outer channels (252) formed in one or both of the manifold (212)and housing (214) between the discharge plenum (228) and back-passchamber (250), and inner channels (251) formed in one or both of themanifold (212) and housing (214) between the back-pass chamber (250) androtor chamber (224); wherein the manifold (212) is sized such that theback-pass chamber (250) volume as a percentage of closed cell (240X)volume falls in a range between about fifty-six percent (56%) andone-hundred-seventeen percent (117%).
 2. The blower (210) of claim 1wherein the main housing (214) comprises a casting.
 3. The blower (210)of claim 2 wherein each manifold (212) comprises a separately removablepart from the main housing (214).
 4. The blower (210) of claim 3 whereineach manifold (212) comprises an independent casting which is boltedonto the main housing (214).
 5. The blower (210) of claim 3 wherein theinner channels (251) are formed by drill holes through the main housing(214) into the rotor chamber (224).
 6. The blower (210) of claim 3wherein the inner channels (251) of each back-pass loop (250-252) areformed by a series of drill holes through the main housing (214) andinto the rotor chamber (224), all aligned linearly on a line parallelwith the rotor axes and equi-distantly spaced.
 7. The blower (210) ofclaim 6 wherein the inner channels (251) are all aligned with an angleof attack that is close to a tangent line with the rotor chamber (224)'sinside wall's curvature, and aimed at the backside of a leading lobe tip(232T) of the closed cell (240X) as the leading lobe tip (232T) crossesthe inner channels (251).
 8. The blower (210) of claim 7 wherein fromwhere a leading lobe tip (232T) of the closed cell (240X) crosses theinner channels (251) to where said leading lobe tip (232T) crosses aledge (244L) of a discharge opening (244) into the discharge plenum(228) comprises between about 30° to 40° angular degrees.
 9. The blower(210) of claim 6 wherein the inner channels (251) for each back-flowloop (250-252) form a cumulative flow area which, when given a specifiedback-pressure in the back-pass chamber (250) and a specifiedunder-pressure in the closed cell (240X) pursuant to the pressure of theinlet plenum (220), said cumulative flow area is selected to fill theclosed cell (240X) with about 100% of the make-up mass of gas in theangular time that a leading lobe tip (232T) of the closed cell (240X)crosses the inner channels (251) to where said leading lobe tip (232T)crosses a ledge (244L) of a discharge opening (244) into the dischargeplenum (228), comprising between about 30° to 40° angular degrees. 10.The blower (210) of claim 9 wherein the outer channels (252) define acumulative flow area and the inner channels (251) define a cumulativeflow area substantially close to the cumulative flow area of the outerchannels (252).
 11. The blower (210) of claim 5 wherein the outerchannels (252) are formed by drill holes through the main housing (214)into the discharge plenum (228).
 12. The blower (210) of claim 1 whereinthe back-pass chamber (250) volume as a percentage of closed cell (240X)volume ranges between about eighty-three percent (83%) andone-hundred-seventeen percent (117%).
 13. The blower (210) of claim 12wherein each rotor (230) consists of three axially-straight lobes (232).14. The blower (210) of claim 12 wherein the back-pass chamber (250)volume as a percentage of closed cell (240X) volume comprisessubstantially close to one-hundred percent (100%).
 15. A method ofmanufacturing a roots-style positive displacement blower (210) with aback-pass loop (251-252), comprising the steps of: providing left andright rotors (230) having lobes (232) spaced by pockets (240); providinga housing (214) having a rotor chamber (224) portion, an inlet plenum(220) portion defining an inlet plenum (220) and a discharge plenum(228) portion; the rotor chamber (224) portion defining a rotor chamber(224) comprising side-by-side partially-overlapping left and rightcylindrical cavities meeting at tangent lines, as well as receiving therotors (230) such that the rotor axes define a rotor plane; thedischarge plenum (228) portion comprising a bell shape extending alongan axis that projects away from the rotor plane and defining a dischargeplenum (228) extending between a discharge opening (244) in the rotorchamber (224) and a discharge port (228P); forming left and right flangeinterfaces on the rotor chamber (224) portion of the housing (214) aswell as forming left and right inner channels (251) in the rotor chamber(224) portion of the housing (214) and extending between interior ports(251P) in the left and right cylindrical cavities respectfully, andexterior ports in the left and right flange interfaces on the rotorchamber (224) portion of the housing (214); forming left and rightflange interfaces of the discharge plenum (228) portion of the housing(214) as well as forming left and right outer channels (252) in thedischarge plenum (228) portion of the housing (214) and extendingbetween interior ports (252P) in the left and right sides respectivelyof the discharge plenum (228), and exterior ports in the left and rightflange interfaces of the discharge plenum (228) portion of the housing(214); providing left and right covers (212), each removably attachingto the housing (214) and covering portions of the flange interfaces onthe rotor chamber (224) portion of the housing (214) as well as portionsof the flange interfaces of the discharge plenum (228) portion of thehousing (214) on the left and right sides respectively of the housing(214), wherein said covers (212) concurrently seal over the exteriorports of the inner and outer channels (251 and 252), respectively, andallow a back-pass flow therebetween underneath said covers (212). 16.The method of claim 15 wherein: said main housing (214) comprises amonolithic casting.
 17. The method of claim 16 wherein: the flangeinterfaces on the rotor chamber (224) portion of the housing (214) arefurther outboard from the axis of the discharge plenum (228) than theflange interfaces of the discharge plenum (228) portion of the housing(214); and each cover (212) comprises an L-shape.
 18. The method ofclaim 15 wherein: each cover (212) comprises a monolithic casting in atubular C-shape, extends between a first interface for mating to theflange interfaces of the discharge plenum (228) portion of the housing(214) and a second interface for mating to the flange interfaces of therotor chamber (224) portion of the housing (214); each cover (212)furthermore defining a back-pass chamber (250) which allows theback-pass flow between the inner and outer channels (251 and 252), andis removably attached to the housing (214) by mechanical fastening. 19.The method of claim 18 wherein: each cover (212) expands from beingrelatively narrower at the first interface to being relatively wider atthe second interface and, moreover, relative to narrower and wider beingtaken along axes parallel to the rotor axes; said left inner channels(251) comprising a series of bore holes axially spread apart on an axisparallel to the rotor axes, the right inner channels (251) comprisingsymmetric opposites of the left inner channels (251); said left outerchannels (252) comprising at least two bore holes axially spread aparton an axis parallel to the rotor axes, the right outer channels (252)comprising symmetric opposites of the left outer channels (252); and theleft inner channels (251) define a cumulative flow area fairly close insize to a cumulative flow area defined by the left outer channels (252).20. The method of claim 16 wherein: the discharge plenum (228) portionfurther comprises a circular ANSI flange encircling discharge port(228P), and the bell shape comprises a six-sided subtendeddiamond-shaped bell.